Even though Sabretooth is the world's most sadistic bastard, and should be utterly irredeemable, writers can't resist the occasional "good guy Sabretooth" what if scenario. Arguably AoA/Exiles is the best depiction of the possibility for him to be a hero.
I think that 616 Sabes is/should be wayyyyyy too far gone for that sort of redemption arc, but the fact that the potential existed at all is something I guess.
I think the two concepts don't necessarily contradict with each other.
There are many "born to be evil" villains, Doomsday, a lot of creatures from Anti-Matter Universe or from Earth-3 etc
They don't necessarily choose to be evil, but are still largely considered to be "evil"
1. Joker
Joker is supposed to be batshit insane but he's really so predictable with all the evil crap that he pulls. Just as a good person might go crazy and and become a murderous lunatic, the craziest thing Joker could do is become a good guy and take on a superhero persona. Or maybe it is only at night and his daytime self wasn't even aware or something. That would be completely f*** insane for him.